I have always thought that on an individual day, we should be more concerned with duty time that with flight time. Flight time is very taxing on the body, and the affect is cumulative, so I would say stick with the 30/7 rule. But on an individual day I would be more concerned with duty time. What would be more tiring, an out and back from JFK to SFO, totalling about 9 hours of flight time and maybe 11 hours on duty, or a six leg day with 8 hours of flight time and 13 hours on duty. Personally I would much rather fly the out and back.